Combustion device

ABSTRACT

A combustion apparatus for use as a boiler wherein RDF is burned as the fuel, or as an incinerator for burning municipal waste. The apparatus is diminished in the quantities of NOx produced, permits a dust collector to achieve an improved efficiency in separating off a free-flowing medium and assures a fluidized-bed furnace of effective fluidization of a medium. The apparatus comprises a fluidized-bed furnace ( 1 ), a cyclone ( 2 ) disposed downstream from the furnace ( 1 ) for separating a free-flowing medium and a combustion residue discharged from the furnace ( 1 ) from combustion gases and collecting these solids, and a medium-residue return channel ( 3 ) provided between the cyclone ( 2 ) and the furnace ( 1 ) for returning the free-flowing medium and the combustion residue collected by the cyclone ( 2  )to the furnace ( 1 ) therethrough. A secondary combustion furnace ( 31 ) is disposed downstream from the cyclone ( 2 ) for completely burning unburned combustibles in the combustion gases egressing from the cyclone ( 2 ) by introducing air into the combustion furnace. A heat recovery unit ( 12 ) is provided at an intermediate portion of the return channel ( 3 ).

FIELD OF THE INVENTION

The present invention relates to combustion apparatus for use as boilerswherein refuse-derived fuel (hereinafter referred to as “RDF”) is usedas the fuel for incinerators for burning municipal waste.

BACKGROUND ART

FIG. 4 shows a combustion apparatus conventionally utilized as a boilerwherein RDF is used as its fuel.

With reference to FIG. 4, the combustion apparatus comprises afluidized-bed furnace 1, a cyclone 2 (dust collector) disposeddownstream from the furnace 1 for separating a free-flowing medium and acombustion residue discharged from the furnace 1 from combustion gasesand collecting these solids, and a medium-residue return channel 3provided between the cyclone 2 and the fluidized-bed furnace 1 forreturning the free-flowing medium and the combustion residue collectedby the cyclone 2 to the furnace 1 therethrough.

The fluidized-bed furnace 1 forms a fluidized bed from sand, or likefree-flowing medium respectfully with primary air and secondary air sentfrom an air preheater. A multiplicity of water tubes (not shown) for aboiler are arranged within the furnace 1 to cover the inner peripheralsurface thereof. These water tubes communicate at their upper ends witha steam drum 6 via an unillustrated header. RDF is supplied from anunillustrated scale conveyor to a hopper 7 in portions of predeterminedquantity and placed into the fluidized-bed furnace 1 from the hopper 7.Noncombustibles and free-flowing medium are drawn off from the lower endof the furnace 1 and separated by a separator 8 disposed below thefurnace 1, and the noncombustibles are discharged from the system, whilethe medium is returned to the furnace 1 by a conveyor 9 and an elevator10.

The medium-residue return channel 3 is bifurcated into two branches atan intermediate portion thereof. One of the branches, 3 a, is providedat intermediate portions thereof with a flow control valve 11 and a heatrecovery unit 12 which are arranged in this order downstream from thecyclone 2. A superheater 13 for the boiler is disposed in the heatrecovery unit 12. Medium transport air is forced into the heat recoveryunit 12. The forward end of the other branch 3 b is opened to thefluidized-bed furnace 1 at a position above the position here the hopper7 is opened to the furnace. The free-flowing medium transported by theconveyor 9 and the elevator 10 is admitted into the branch 3 b at anintermediate portion thereof.

A heat recovery column 14 is disposed downstream from the cyclone 2.This column 14 has in its interior a first flue 15 for passingcombustion gases, discharged from the cyclone 2, from above downwardtherethrough, and a second flue 16 in communication with the lower endof the first flue 15 for passing the combustion gases from below upwardtherethrough. The inner peripheral surface of the first flue 15 iscovered with a multiplicity of boiler water tubes (not shown), the upperends of which also communicate with the steam drum 6 via anunillustrated header. Arranged within the second flue 16 are a boilereconomizer 17 for passing therethrough water forwarded from a deaerator,and two superheaters 18, 19 connected in series with the superheater 13in the heat recovery unit 12 for passing steam sent from the steam drum6. A water injector 20 for adjusting the temperature and pressure of thesteam by injecting water is disposed between the two superheaters 18, 19within the second flue 16, also between the lower superheater 19 in theflue 16 and the superheater 13 in the heat recovery unit 12.

A temperature reduction column 21 and a bag filter 22 are arrangeddownstream from the heat recovery column 14. The combustion gasespassing through the heat recovery column 14 are further reduced intemperature by the heat reduction column 21. The bag filter 22 serves tocollect hydrogen chloride, sulfur oxides, soot and dust from thecombustion gases. Slaked lime, or like neutralizing agent, and areaction assisting agent, are added to the combustion gases at aposition upstream from the filter. After passing through the bag filter22, the combustion gases are released into the atmosphere through astack.

Fly ash is discharged from the lower ends of the heat recovery column14, temperature reduction column 21 and bag filter 22 and sent to a flyash treating unit (not shown).

With the combustion apparatus thus constructed, RDF is sent into thefluidized-bed furnace 1 by the hopper 7. In the furnace 1, thefree-flowing medium is formed into a fluidized bed with primary air andsecondary air, and RDF is burned in the fluidized bed. The unburnedcombustibles are almost completely burned until the combustion gases andthe medium enter the cyclone 2 from the upper end of the furnace 1. Themedium and combustion residue discharged from the furnace 1 areseparated from the combustion gases and trapped in the cyclone 2, passedthrough the two branches 3 a, 3 b of the return channel 3 and returnedto the furnace 1. While passing through the branch 3 a having the heatrecovery unit 12, the medium and the residue have their heat transferredto steam flowing through the superheater 13, whereby the steam issuperheated. The temperature of the furnace 1 can be lowered byadjusting the amount of medium transport air to be forced in and theopening degree of the flow control valve 11 and thereby increasing thequantities of medium and residue to be passed through the branch 3 ahaving the heat recovery unit 12. Conversely, the temperature of thefurnace 1 can be raised by reducing the quantities of medium and residueto be passed through the branch 3 a having the heat recovery unit 12.

The combustion gases flowing out of the cyclone 2 enter the heatrecovery column 14, flow down the first flue 15 first and consequentlyhave their heat transferred to boiler water in the water tubes providingthe wall of the flue, whereby the boiler water in the water tubes isheated and evaporated, and the temperature of the combustion gases islowered. The combustion gases then flow upward through the second flue16 and have their heat transferred to the boiler water in the watertubes constituting the wall of the flue, to the steam in the twosuperheaters 19, 18 and to the water in the economizer 17, whereby theboiler water in the water tubes is heated and evaporated, the steam issuperheated, and the water in the economizer 17 is preheated to resultin a drop in the temperature of the combustion gases.

The combustion gases subsequently flow into the temperature reductioncolumn 21 and have their temperature further reduced. Slaked lime, orlike neutralizing agent, and a reaction assisting agent, are thereafteradded to the combustion gases, and the resulting mixture is led into thebag filter 22, in which hydrogen chloride, sulfur oxides, soot and dustare removed. The gases separated off are then released into theatmosphere.

On the other hand, the water sent from the deaerator and preheatedduring passage through the economizer 17 is admitted into the steam drum6 and thereafter further heated within the water tubes of thefluidized-bed furnace 1 and the heat recovery column 14 constituting aboiler water circulation circuit to become a steam-water mixture, whichis then sent to the steam drum 6 again. The steam is separated off inthe steam drum 6, passed through the three superheaters 18, 19, 13 insuccession and superheated with the heat of the combustion gases duringpassage through the superheaters 18, 19 and with the heat of thefree-flowing medium during passage through the superheater 13. Thesuperheated steam is sent to a steam turbine.

However, the conventional combustion apparatus has the problem thatalmost complete combustion of RDF in the fluidized furnace 1 increasesthe internal temperature of the furnace 1 to a considerably high leveland necessitates supply of secondary air at a high rate to result in thepresence of a large amount of oxygen, consequently producing largequantities of NOx with the nitrogen contained in RDF and the nitrogenafforded by the air. Further, since the combustion gases flowing intothe cyclone 2 have a considerably high temperature, for example, of atleast about 800° C., the apparatus has another problem that the ashresulting from incineration and contained in the gases is partly meltedand adheres to the cyclone 2 to entail an impaired medium separationefficiency. Another problem is also encountered in that the molten ashadheres to the free-flowing medium, causing faulty fluidization in thefurnace 1. An attempt to reduce the temperature of the furnace 1 topreclude these problems will entail the problem of giving rise toincomplete combustion to produce increased amounts of unburnedcombustibles.

An object of the present invention is to overcome the foregoing problemsand to provide a combustion apparatus which is diminished in thequantities of NOx produced, further permitting a cyclone, or like dustcollector, to achieve an improved efficiency in separating off afree-flowing medium and assuring a fluidized-bed furnace of effectivefluidization.

DISCLOSURE OF THE INVENTION

The present invention provides a combustion apparatus comprising afluidized-bed furnace, a dust collector disposed downstream from thefurnace for separating a free-flowing medium and a combustion residuedischarged from the furnace from combustion gases and collecting themedium and the residue, a medium-residue return channel provided betweenthe dust collector and the fluidized-bed furnace for returning thefree-flowing medium and the combustion residue collected by the dustcollector to the furnace therethrough, and a secondary combustionfurnace disposed downstream from the dust collector for completelyburning unburned combustibles in the combustion gases egressing from thedust collector by introducing air into the combustion furnace.

When the apparatus is thus constructed, the portion of combustibles leftincompletely burned in the fluidized-bed furnace and contained in thecombustion gases from the dust collector can be completely burned in thesecondary combustion furnace in the presence of air introduced therein.This makes it possible to give a relatively low internal temperature tothe fluidized-bed furnace and to reduce the rate of supply of oxygen,consequently permitting formation of smaller quantities of NOx than inthe conventional apparatus. Since the fuel is not completely burned inthe fluidized-bed furnace, the combustion gases flowing into the dustcollector have a relatively low temperature, preventing incineration ashfrom melting in the dust collector and consequently enabling the dustcollector to separate the free-flowing medium from the combustion gaseswith a higher efficiency than in the conventional apparatus.

Because molten incineration ash is unlikely to adhere to the medium,faulty fluidization within the fluidized-bed furnace is avoidable. Theunburned combustibles contained in the combustion gases from the dustcollector can further be burned completely in the secondary combustionfurnace.

The combustion apparatus of the present invention may further comprise aheat recovery unit disposed at an intermediate portion of themedium-residue return channel for recovering heat from the free-flowingmedium and combustion residue trapped in the dust collector.

Heat can then be recovered from the heated medium and combustion residuefor effective use.

Preferably, the heat recovery unit is, for example, a superheater for aboiler.

In this case, the free-flowing medium and combustion residue in thefluidized-bed furnace flow into the dust collector along with thecombustion gases, and the residue is retained in the collector for asufficient period of time before it is returned as separated from thegases to the fluidized-bed furnace, whereby the residue isdechlorinated. This precludes high-temperature corrosion of thesuperheater due to the presence of chlorine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the overall construction of a combustionapparatus of the invention for use as a boiler wherein RDF is used asthe fuel;

FIG. 2 is a fragmentary diagram in horizontal section showing a modifieddust collector as a substitute for a cyclone;

FIG. 3 is a diagram showing the overall construction of a combustionapparatus of the invention for use as an incinerator for burningmunicipal waste; and

FIG. 4 is a diagram showing the overall construction of a conventionalcombustion apparatus for use as a boiler wherein RDF is used as thefuel.

BEST MODE OF CARRYING OUT THE INVENTION

The best mode of carrying out the invention will be described below withreference to the drawings.

FIG. 1 shows the overall construction of a combustion apparatus of theinvention for use as a boiler wherein RDF is used as the fuel. In FIGS.1 and 4, like parts are designated by like reference numerals and willnot be described repeatedly.

The fluidized-bed furnace 1 of the combustion apparatus shown in FIG. 1does not have as large a space as in the apparatus of FIG. 4 but isbasically the same as that of FIG. 4 in construction. However, thecombustion apparatus comprises a secondary combustion furnace 31disposed downstream from the cyclone 2 and upstream from the heatrecovery column 14. The secondary combustion furnace 31 is supplied withtertiary air and completely burns the unburned combustibles in thecombustion gases delivered from the fluidized-bed furnace 1 via thecyclone 2. With the exception of this feature, the apparatus is the sameas the one shown in FIG. 4.

RDF is placed by the hopper 7 into the fluidized-bed furnace 1 of thecombustion apparatus thus constructed and burned in the fluidized bedwithin the furnace 1 as is the case with the apparatus of FIG. 4.

The fuel is burned in the furnace 1 at a smaller rate of supply ofsecondary air than when it is completely burned therein, i.e., at alower oxygen concentration, and is not completely burned, so that thetemperature of combustion is lower, consequently reducing the quantitiesof NOx to be produced, especially NOx responsible to the nitrogencontained in the RDF.

Combustion gases containing unburned combustibles and the free-flowingmedium containing a combustion residue flow into the cyclone 2 from theupper end of the fluidized-bed furnace 1. As in the apparatus of FIG. 4,the medium and combustion residue are separated off and trapped in thecyclone 2 and returned to the furnace 1 through the two branches 3 a, 3b of the medium-residue return channel 3. The steam flowing through thesuperheater 13 is superheated while the medium and residue pass throughthe branch 3 a.

The period of time from the charging of RDF into the furnace 1 until themedium and combustion residue are separated off in the cyclone 2, andthe internal temperature of the furnace 1 and the cyclone 2 are sodetermined that the chlorine contained in the combustion residue of theRDF is thermally removed. More specifically, the time is 1 to 2 seconds,and the temperature is 650 to 800° C. Accordingly, even if the steamflowing through the superheater 13 in the heat recovery unit 12 of thebranch 3 a has a high temperature, for example, of not lower than 500°C., the corrosion of the superheater 13 at high temperatures can beprecluded. As a result, the steam to be sent to a steam turbine can beset at a temperature of at least 500° C., enabling the steam turbinegenerating plant to achieve a remarkably improved efficiency.

On the other hand, since incineration ash starts to melt at about 800°C., the condition described above obviates the likelihood of moltenincineration ash adhering to the interior of the cyclone 2, consequentlyprecluding an impairment in the efficiency of separating the medium andcombustion residue from the combustion gases. Further because moltenincineration ash is unlikely to adhere to the free-flowing medium,faulty fluidization is avoidable.

The combustion gases egressing from the cyclone 2 flow into thesecondary combustion furnace 31, into which tertiary air is introducedto completely burn the unburned combustibles in the gases. In this case,the combustion gases have a temperature of about 850 to about 900° C.and are retained in the furnace for about 2 seconds. This temperaturerange is low for the formation of NOx from the nitrogen in air, thusserving to inhibit NOx.

The resulting combustion gases flow out of the secondary combustionfurnace 31, pass along the heat transfer surfaces of the boiler as inthe apparatus of FIG. 4, have their temperature reduced in thetemperature reduction column 21, and then flow into the bag filter 22,in which hydrogen chloride, sulfur oxides, soot and dust are removed.The gases are thereafter released into the atmosphere via a stack.

On the other hand, the superheated steam to be fed to the steam turbineis obtained by the same process as in the apparatus of FIG. 4.

FIG. 2 is a plan view of a modified dust collector which is to bedisposed downstream from the fluidized-bed furnace 1 for separating thefree-flowing medium and combustion residue discharged from the furnace 1from the combustion gases and trapping these solids.

With reference to FIG. 2, the dust collector 35 comprises a multiplicityof medium collision plates 36 provided inside an unillustrated housingin a staggered arrangement when seen from above. Each plate 36 for thefree-flowing medium and combustion residue to collide with ischannel-shaped and has an open side facing toward the upstream side withrespect to the flow of combustion gases when seen from above. The plateis left open at its upper and lower ends.

In the case of this dust collector 35, the medium and residue strikingagainst the collision plates 36 flow down the plates 36 and aredischarged from the lower end of the housing.

FIG. 3 shows the overall construction of a combustion apparatus of theinvention for use as an incinerator for burning municipal waste. InFIGS. 3 and 1, like parts are designated by like reference numerals andwill not be described repeatedly.

In the case of the combustion apparatus of FIG. 3, the heat recoverycolumn included in the apparatus of FIG. 1 is not disposed between thesecondary combustion furnace 31 and the temperature reduction column 21.Further the heat recovery unit 12 of the branch 3 a of themedium-residue return channel 3 has a steam generator or like heatexchanger 40 for heat recovery in place of the boiler superheater. Theapparatus has none of the components needed for the boiler, i.e., nowater tubes along the inner periphery of the fluidized-bed furnace 1,and no steam drum. With the exception of these features, the apparatushas the same construction as the one shown in FIG. 1.

With the combustion apparatus thus constructed, municipal waste placedinto the fluidized-bed furnace 1 by the hopper 7 is burned, and thefree-flowing medium and combustion residue separated from the combustiongases and trapped in the cyclone 2 are returned to the furnace 1, in thesame manner as in the apparatus of FIG. 1.

It will be appreciated that the combustion apparatus of the invention issuitable for use as a boiler wherein RDF is used as the fuel, or as anincinerator for burning municipal waste.

What is claimed is:
 1. A combustion apparatus comprising a fluidized-bedfurnace having fuel delivery means and a primary combustion air supplymeans communicating with said fluidized-bed furnace for introducingcombustion air to said fluidized-bed furnace in amounts less than thatrequired for achieving complete combustion of the fuel delivered to saidfurnace a dust collector disposed downstream from the furnace forseparating a free-flowing medium and a combustion residue dischargedfrom the furnace from combustion gases and collecting the medium and theresidue, a medium -residue return channel provided between the dustcollector and the fluidized-bed furnace for returning the free-flowingmedium and the combustion residue collected by the dust collector to thefurnace therethrough, and a secondary combustion furnace disposeddownstream from the dust collector, said secondary combustion furnacehaving combustion air supply means for introducing combustion air inamounts required for completely burning unburned combustibles in thecombustion gases egressing from the dust collector.
 2. A combustionapparatus according to claim 1 wherein a heat recovery unit is deposedat an intermediate portion of the medium-residue return channel forrecovering heat from the free-flowing medium and the combustion residuecollected by the dust collector.
 3. A combustion apparatus comprising afluidized-bed furnace, a dust collector disposed downstream from thefurnace for separating a free-flowing medium and a combustion residuedischarged from the furnace from combustion gases and collecting themedium and the residue, a medium-residue return channel provided betweenthe dust collector and the fluidized-bed furnace for returning thefree-flowing medium and the combustion residue collected by the dustcollector to the furnace therethrough, and a secondary combustionfurnace disposed downstream from the dust collector for completelyburning unburned combustibles in the combustion gases egressing from thedust collector by introducing air into the combustion furnace, wherein aheat recovery unit is disposed at an intermediate portion of themedium-residue return channel for recovering heat from the free-flowingmedium and the combustion residue collected by the dust collector, theheat recovery unit being a superheater for a boiler.